1. Field of the Invention
The present invention relates to a nondestructive and portable adhesive tape for detecting the various stages of metal and coating degradation by electrochemical corrosion. More specifically, the present invention relates to a portable electrochemical sensor which is utilized under field or actual environmental conditions for detecting coating and material or substrate degradation by electrochemical corrosion of both small and large coated or painted structures, as well as uncoated metal structures, thereby permitting detection of substrate degradation and electrochemical corrosion well before serious deterioration of the substrate or structure has occurred. The present invention is comprised of a conductive foil tape with a conductive, pressure-sensitive adhesive and another conductive foil tape comprises a non-conductive, pressure-sensitive adhesive. The conductive tape serves as the sensing element or device. The non-conductive tape serves as the lead between the sensing element and the point of measurement. Alternatively, the first tape can serve as both the sensing element and the lead. Used concurrently with one another through overlapping of the tapes in order to make electrical contact, both foil tapes serve as a detection sensor of substrate or coating degradation. Utilizing either configuration of the tapes, the sensor can be attached to a substrate of arbitrary shape, composition or orientation with the substrate serving as an electrochemical medium for physical and chemical analysis by exposure to selected external agents, physical or chemical, causing electrochemical alterations in the substrate's material, and changes in the chemical properties of the materials are then sensed and monitored for changes in the electrochemical impedance spectroscopy (EIS) spectra to read out the recorded information in correlation with the analyzed phenomenon or process for storage, monitoring or control purposes.
2. Prior Art
A major goal in the electrochemical field has long been to create a sensor which could be utilized in field or service conditions to detect corrosion and adhesion on metal structures of any size before significant degradation has occurred. Evaluation of materials and coatings and the determination or prediction of corrosion performance of both painted and uncoated metal structures or specimens under ambient field or service conditions has traditionally involved visual comparisons which are subjective and require blistering, rusting, or other advanced stages of degradation. The use of laboratory techniques, such as EIS (or AC impedance) has been used to understand and predict corrosion performance during immersion exposures in different electrolytes was limited to small structures or witness specimens that could be immersed, small sections of material cut from large structures, or attachment to the structure of a clamp-on liquid cell in which a liquid or semi-liquid electrolyte and remote counter and reference electrodes were contained.
The immersion of small specimens requires either the destructive sampling of a large structure or the use of witness specimens prepared differently than the actual structure of interest (although the witness specimens and the structure may be prepared at the same time, inherent differences in coating small and large surfaces and inadvertent differences caused by operator error will prevent the witness specimens from being exactly the same as the structure). Additionally, witness specimens will be exposed to slightly different environmental conditions compared to a large structure. Furthermore, the immersion in an electrolyte is not necessarily the exposure condition relevant to the structure being inspected.
Inspection of a large structure using conventional EIS methodologies required complete immersion or use of a clamp-on cell. Such cells would be filled with a liquid or semi-liquid electrolyte (e.g., Kihira el al, U.S. Pat. No. 4,806,849) or a spongy medium impregnated with a liquid electrolyte (e.g., Kondou el al, U.S. Pat. No. 5,221,893) with remote electrodes immersed in the electrolyte or in intimate contact with the electrolyte-impregnated sponge. These cells required an accessible, flat, smooth, and horizontal area. The set-up was considered to be time consuming and had to be performed for each measurement. Corrosion was detected only directly under the cell and use of the cell actually caused artifactual damage to the coating in many instances because of exposure to the electrolyte during measurement.
Davis et al, U.S. Pat. No. 5,859,537, recently developed a painted electrode sensor which eliminates many of the problems discussed above. The actual structure is being inspected without exposure to an extrinsic electrolyte. Measurements are possible under most natural or accelerated conditions and material and coating degradation are detectable from the very early stages. However, the Davis et al, sensor requires an electrode to be permanently painted onto the structure and is time-consuming for all the fabrication steps to be completed. It is not suitable for structures in which appearance or aerodynamics preclude an attached sensor. The sensor can induce artifactual damage in a small class of materials, primarily porous coatings.
Presently, there does not exist a non-invasive corrosion sensing device consisting of an adhesive tape which: (1) provides early detection of both substrate and coating degradation; (2) evaluates substrate or material degradation on structures of any size or composition, under actual conditions, as well as under aggressive corrosive conditions; and (3) can be installed or removed in a expedient and relatively facile manner. Further, no corrosion sensing device currently exists which does not require permanent or fixed attachment to the substrate being evaluated for structural degradation.